1. Field of the Invention
The present invention relates to optically active compounds and liquid crystal compositions comprising said compounds. The compounds and compositions of the present invention show a ferroelectric liquid crystal phase, and accordingly are useful as electrooptic switching elements (e.g. liquid crystal display devices) and can be applied to liquid crystal optical modulators.
2. Related Art
Liquid crystal display devices have various excellent features such as low-voltage operability, lower electricity consumption, being thin and light-weight, being a non-emissive type and easy on the eye, etc. Accordingly, they are in wide use as various display devices. Liquid crystal display devices using a nematic liquid crystal operating in the so-called twisted nematic mode (TN mode) are in use currently. However, the display devices using this nematic liquid crystal have the drawback of being very slow in response as compared to luminescent type display devices such as CRT, EL and the like. Accordingly, when the liquid crystal display devices using a nematic liquid crystal are applied in a large-scale display device capable of displaying a large amount of information, it is impossible to obtain a display of good contrast because of insufficient threshold characteristic. Thus, the liquid crystal display devices using a nematic liquid crystal have had a limitation for wide application. There has recently been developed a liquid crystal display device using a nematic liquid crystal operating in the so-called super twisted nematic mode (STN mode) or SBE and capable of giving a display of improved contrast because of improved threshold characteristic. Even in this STN mode liquid crystal display device, however, the response is not significantly improved, and therefore said device has a limitation for application to displays capable of displaying a still larger amount of information. Hence, various attempts have been made to develop a new liquid crystal display system capable of giving excellent response.
Ferroelectric liquid crystals have a memory characteristic and give a high speed response, and accordingly their application to large-scale displays is highly expected. As liquid crystals having ferroelectric properties, there are known those showing a chiral smectic C phase, a chiral smectic H phase, a chiral smectic J phase, etc.
Of these ferroelectric liquid crystals, those showing a chiral smectic C phase were first synthesized in 1975 by R. B. Meyer et al.; one typical example thereof is 2-methylbutyl 4-(4'-decyloxybenzylideneamino)cinnamate (hereinafter abbreviated to DOBAMBC) [J. Physique, 36, L-69 (1975)].
A thin film liquid crystal cell was prepared using DOBAMBC and was found to have a high speed response in the order of .mu.sec [N. A. Clark et al., Appl. Phys. Lett., 36, 89 (1980)]. Since that time, there was started the development of optical modulation devices (e.g. liquid crystal display devices, photo-printer heads) using a ferroelectric liquid crystal showing a chiral smectic C phase (hereinafter may be referred to simply as "ferroelectric liquid crystal").
As a result, a number of ferroelectric liquid crystal compounds showing a chiral smectic C phase have been developed since then, and various ferroelectric liquid crystal compounds are already known. However, no ferroelectric liquid crystal compound is found yet which has satisfactory reliability and capability for use in large-scale displays, etc.
In order for a ferroelectric liquid crystal to be practically used in a liquid crystal display device, etc., the liquid crystal must be superior in high speed response, orientation, memory characteristic, characteristic of threshold voltage, temperature dependences of these properties, etc. Also, the ferroelectric liquid crystal is required to show a chiral smectic C phase over a wide temperature range so that it can operate within a sufficiently wide temperature range including room temperature, and further to have excellent physical and chemical stabilities.
Of these requirements, particularly important are physical and chemical stabilities, high speed response, large spontaneous polarization necessary for the expression of memory characteristic, large pitch of cholesteric structure for achieving good orientation, and wide temperature range shown by chiral smectic C phase.
None of the so far developed ferroelectric liquid crystal compounds satisfy almost all of the above requirements. For example, the above-mentioned DOBAMBC, being a SCHIFF's base type liquid crystal, has a problem in chemical stability to water, light, etc. and moreover is low in spontaneous polarization (4 nC/cm.sup.2 or less).
Ester type liquid crystals have been reported as a chemically stable ferroelectric liquid crystal compound. These liquid crystals, however, are not satisfactory because they neither have a sufficiently large spontaneous polarization nor a sufficiently wide temperature range for chiral smectic C phase.
In order to obtain a larger spontaneous polarization, there have been synthesized compounds having two asymmetric carbon atoms in the optically active group which is essential for the expression of chiral smectic C phase.
These compounds include, for example, liquid crystal compounds having a dichiral epoxide side chain [David M. Walba et al., Journal of American Chemical Society, 108, 7424 (1986)], and liquid crystal compounds having a halogen atom and a methyl group on two adjacent asymmetric carbon atoms [cf. e.g. JP-A-168780/1985, 218358/1985, 68449/1986, 40/1987, 46/1987, 103043/1987, 111950/1987, 142131/1987, 175443/1987].
A typical example of the above liquid crystal compounds is 4'-octylcarbonyloxy-4-biphenyl (S)-3-methyl-2-chloropentanoate [JP-A-68449/1986]. This liquid crystal compound has a very large spontaneous polarization of 190 nC/cm.sup.2, but, being an aliphatic chloro compound, has poor chemical stability. Hence, there has been synthesized 4'-octylcarbonyloxy-4-[(S)-2-methoxy-(S)-3-methylpentyloxycarbonyl]bipheny l [JP-A228036/1987]. This compound has excellent chemical stability but has a small spontaneous polarization of 17 nC/cm.sup.2.
As described above, according to the prior art there has been found yet no ferroelectric liquid crystal compound which satisfies requirements such as physical and chemical stabilities, high speed response, large spontaneous polarization (necessary for the expression of memory characteristic) and the like.